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<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
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<body>
    <script>
        var a = 1;
        var b = a++;
        console.log(a, b); //2,1

        var c = 1;
        var d = ++c;
        console.log(c, d) //2,2
    </script>
</body>

</html>